Sensitization or antibody production to alloantigens present in blood transfusions and organ allografts precludes transplantation of as many as 25% of potential kidney graft recipients. Our objective is to develop a rational approach to specific desensitization of HLA class I-sensitized patients by means of antigen receptor-specific reagents. Our approach is based, in part, on our preliminary data supporting the hypothesis that human B cell responses to HLA class I are directed to relatively few polymorphic epitopes and are idiotypically restricted in both the 10 (IgM) and 20 (IgG) response repertoire. The rationale for the project is that by developing anti-idiotypic (Ab-2) reagentS specific for the restricted VH- and VL-encoded idiotopes on anti-HLA class I B cells, as well as soluble divalent and decavalent forms of HLA class I recombinant fusion proteins, specific therapy to eliminate these B cells can be developed. The specific alms are: l) to further characterize the human B cell response to HLA-A2 at both protein (epitope and idiotype) and DNA (V region gene utilization) levels with the goal of identifying new targets for receptor-based therapy; 2) to develop and test monoclonal and polyclonal Ab-2 reagents as immunotoxins in the hu/SCID in vivo model system for anti-HLA-A2 response developed in our laboratory. In addition to developing and testing new Ab-2 reagents and immunotoxins, we propose to test a novel class of HLA class I/Ig fusion proteins that resemble anti-idiotype in terms of valency and ability to specifically bind to B cells bearing receptors for HLA-A2; and 3) to use the hu/SCID model to evaluate the role of HLA-A2 peptide-specific T helper (TH) cells in the generation of antibody responses to HLA-A2 and to determine the cell subsets required to induce Tn-dependent B primary and memory cell IgG responses in vivo. The possible therapeutic effectiveness of TH cell inhibition (e.g., anti-CD4 mAb, high dose peptide) will be evaluated using the hu/SCID model. The three parts of the project form an integrated approach to analysis of the processes of sensitization and de-sensitization to MHC class I alloantigens, with an emphasis on identifying the cells and molecules involved in both. The overall goal is to develop an effective therapy to reverse the sensitized state, and prevent its re-emergence.